1. Field of the Invention
This invention relates to a method for the production of an acrylate and an acrylate-containing polymer. More particularly, it relates to a method for the production of an acrylate advantageously usable for the production of an acrylate-containing polymer with a reduced residual monomer content and an acrylate-containing polymer by the use of the acrylate.
2. Description of the Prior Art
The water-soluble acrylate-containing polymers obtained by polymerizing acrylate-containing monomer components include sodium polyacrylate, acrylamide-sodium acrylate copolymer, and acrylic acid-sodium acrylate copolymer, for example. These water-soluble polymer compounds are used as flocculants for water treatment, additives for drilling mud, food additives, and viscosity enhancers, for example.
The water-swellable acrylate-containing polymers obtained by polymerizing acrylate-containing monomer components include crosslinked acrylate polymer, partially neutralized acrylic acid-crosslinked polymer and starch-acrylate graft polymer, for example. They are used as sanitary absorbents such as sanitary napkins and disposable diapers, water-retaining agents for agriculture, horticulture, and afforestation, and freshness-retaining films for foodstuffs, for example. The demand for the polymers in these fields is expected to grow further in the future.
The acrylate-containing polymer is obtained by polymerizing an acrylate or an acrylate-containing a monomer component. Various methods have been proposed for the production of such acrylate-containing polymers as in Japanese Patent Laid-Open SHO 58(1983)-108,212, Japanese Patent Laid-Open SHO 58(1983)-71,907, Japanese Patent Laid-Open SHO 53(1978)-46,389, Japanese Patent Laid-Open SHO 56(1981)-93,716, Japanese Patent Laid-Open SHO 58(1983)-49,714, Japanese Patent Laid-Open SHO 61(1986)-166,809, Japanese Patent Laid-Open SHO 62(1987)-22,811, Japanese Patent Laid-Open SHO 61(1986)-166,809, and Japanese Patent Laid-Open SHO 62(1987)-22,811, for example.
The acrylates which are used as raw materials in these methods of production are generally obtained by a method which comprises adding a basic substance in a prescribed amount to an aqueous acrylic acid solution or adding acrylic acid to an aqueous solution having a prescribed amount of the basic substance dissolved therein.
Acrylate-containing polymers which are produced by using acrylates obtained by the method just mentioned generally have residual monomers remaining therein in large amounts. Particularly, in the case of a water-swellable polymer which is obtained by using a large amount of an initiator during the course of production, the produced polymer generally has an residual monomer content approximately in the range of 700 to 3,000 ppm. It goes without saying that the water-swellable acrylate-containing polymers having such large residual monomer contents as described above are undesirable as materials for a wide variety of absorbent articles represented by sanitary goods. In these applications, the polymers have the possibility of being brought into contact with human skins or even being absorbed into human bodies. When they are discarded and suffered to flow out into environments, they have the possibility of finding their way into drinking water, for example. In the circumstances, the desirability of lowering residual monomer contents in such polymers has come to command a recognition in recent years.
Generally as means of decreasing residual monomer contents in hydrophilic polymers, methods which effect a decrease in residual monomer contents of polymers by adding ammonia or amine (Japanese Patent Publication SHO 33(1958)-2,646 and Japanese Patent Laid-Open SHO 50(1975)-40,689) or adding a sulfite or a hydrogen sulfite (U.S. Pat. No. 2,960,486 and Japanese Patent Laid-Open SHO 55(1980)-135,110) to the polymers thereby inducing addition of the compounds to the residual monomers, methods which attain a decrease in the residual monomer contents of polymers by additionally using a low-temperature decomposing type or high-temperature decomposable polymerization initiator (Japanese Patent Publication SHO 50(1975)-42,280, Japanese Patent Laid-Open SHO 59(1984)-133,205, and Japanese Patent Laid-Open SHO 53(1978)-141,388) or using a redox catalyst in conjunction with an azo compound initiator (Japanese Patent Laid-Open SHO 50(1975)0-96,689 and Japanese Patent Publication SHO 47(1972)-26,430) thereby inducing polymerization of the residual monomers, and a method which accomplishes a decrease in residual monomer contents of polymers by causing the residual monomers to be decomposed by the use of a microorganism (Japanese Patent Publication SHO 60(1985)-29,523), for example, have been proposed.
Indeed the addition of ammonia, amine, sulfite, or a hydrogen sulfite is fairly effective in decreasing the residual monomer contents of the polymers, this effect is not enough when this addition is made in a small amount. This addition further entails the problem that the compound so added is toxic. The methods resorting to additional use of a polymerization initiator are not sufficiently effective in decreasing the residual monomer contents. The method relying on the action of a microorganism has the problem that the operation lacks high commercial feasibility.
An object of this invention, therefore, is to provide a novel method for the production of an acrylate and an acrylate-containing polymer.
Another object of this invention is to provide a method for production of an acrylate-containing polymer having the residual monomer content thereof decreased without a sacrifice in the substantial characteristics of the acrylate-containing polymer and a method for efficient production of an acrylate advantageously usable for the production of the polymer.
Yet another object of this invention is to provide a water-swellable acrylate-containing polymer excelling in absorption capacity, possessing only a low water-soluble content and a low residual acrylate monomer content in the cross-linked polymer, and serving advantageously as sanitary materials such as sanitary napkins and disposable diapers, as water-retaining agents for agriculture, horticulture, and afforestation, and as materials for a wide range of absorbent articles.